Flexible circuit boards that include a printed circuit on a flexible substrate have heretofore been applied to a variety of electronic devices. The flexible circuit board typically includes a substrate, a circuit printed on the substrate, and a circuit protective layer for protecting the circuit.
For such a circuit protective film and a substrate for flexible circuit boards, heat-resistant resin films made of polyimide or other resin are widely used. Polyimides are prepared for example by dehydrative cyclization of polyamic acids which are produced by reaction between diamines and tetracarboxylic acid dianhydrides in solvent. The characteristics of polyamic acids and polyimides are governed by the kinds and combinations of the source diamines and tetracarboxylic acid dianhydrides. Among different types of polyimides, such polyimides are known that exhibit superior heat resistance and/or dimensional stability.
Aromatic polyimide materials prepared using, among different types of tetracarboxylic acid dianhydrides, aromatic tetracarboxylic acid dianhydrides exhibit superior heat resistance, mechanical properties, and flame retardancy. While these polyimides are superior in heat resistance, they may suffer from a drawback in their processability due for example to poor solubility in solvents or extremely high melting points.
Recently, slide cellular phones have become a focus of attention for their excellent features including design and are replacing conventional foldable cellular phones. A slide cellular phone refers to a cellular phone that includes a key pad unit (also referred to as a “main board unit”) and a separate display unit on the key pad unit, so that the user can slide away the display unit to expose the key pad unit. To establish electrical connection between the key pad unit and display unit in the slide cellular phone, the above-described flexible circuit board is used. The flexible circuit board is bonded at one end to a portion of the key pad side electrical circuit board and at the other end to a portion of the display side electrical circuit board. Accordingly, the flexible circuit board is disposed being bent in U shape at a predetermined curvature radius. Along with opening and closing of the display unit, the U-shaped flexible circuit board is caused to slide back and forth repeatedly (see e.g., Patent Literature 1). It is therefore required for the flexible circuit board to resist breakage during the above-described reciprocating motion (called “sliding flexure”), i.e., to have excellent flexure fatigue life. More recently, as the slide cellular phones are becoming thinner, the size of a gap formed when the flexible electrical circuit board is bent in U shape, which size can be approximated by the diameter of curvature, is becoming smaller. Sliding conditions become more stringent with reducing gap size; therefore, circuit protective films for flexible circuit boards that can meet the stringent conditions have been required.
Flexible circuit board protective films known in the art that exhibit superior flexure fatigue life include circuit protective films prepared by bonding polyimide to substrates using an epoxy resin adhesive. However, a single layer of polyimide or epoxy resin was unable to be used as the circuit protective film. Although interlayer insulation materials containing polyimide or epoxy resin layer are known, they do not have good flexure fatigue life. Moreover, single-layered circuit protective layers consisting of a mixture of polyimide and acrylic resin or a mixture of epoxy resin and acrylic resin are known. These circuit protective layers, however, do not have good flexure fatigue life.
As heat-resistant resins that have characteristics similar to those of polyimides, polyoxazolidones are known which are produced by reacting diisocyanate compounds with epoxy resins (see, e.g., Patent Literatures 2-5). Methods of high-yield production of a high-molecular polyoxazolidone are also proposed wherein a dicarbamate compound, obtained by blocking the isocyanate of a diisocyanate compound, is reacted with an epoxy resin in the presence of a phosphazenium catalyst.